mongoose findByIdAndUpdate instead of save - node.js

Question

I'm trying to update the value of an element inside an array of my document.
The common way of update via save works as expected, but trying to use an static method, like findByIdAndUpdate doesn't work as expected.
Here bellow I paste the code I'm currently using:
var UserSchema = new mongoose.Schema({
nickname: { type: String, trim: true},
username: { type: String, trim: true },
notifications: [{
a: {
_id: { type: mongoose.Schema.Types.ObjectId, ref: 'x' },
x: { type: mongoose.Schema.Types.ObjectId, ref: 'y' }
},
b: {
_id: { type: mongoose.Schema.Types.ObjectId, ref: 'y' },
x: { type: mongoose.Schema.Types.ObjectId, ref: 'y' }
},
read: { type: Number, default: 0 }, // 0 - Unread, 1 - read
ts: { type: Date, default: Date.now }
}]
}, { timestamps: { createdAt: 'created_at' } });
// This works as expected
UserSchema.statics.rNSave = function (user_id, notification_id) {
var vm = this;
return new Promise(function (resolve, reject) {
vm.findById(user_id, function (err, data) {
if (err) {
reject(new Error(err));
} else {
var notifications = data.notifications, i = 0;
for (i; i < notifications.length; i += 1) {
if (data.notifications[i]._id.toString() === notification_id) {
data.notifications[i].read = 1;
data.save({ validateBeforeSave: false }, function (err, updatedData) {
if (err) {
reject(new Error(err));
} else {
resolve();
}
});
return;
}
}
return reject('Error');
}
});
});
};
// This one is not working
UserSchema.statics.rNStatic = function (user_id, notification_id) {
return this.findByIdAndUpdate({ _id: user_id, notifications: { $elemMatch: { _id: notification_id }}}, { $set: { 'notifications.$.read': 1 }}).exec();
};
Any help with this?
Thanks in advice.